The rapid growth of the "information highway" has created the need for high speed, low-cost techniques for transmitting data to and from homes, small businesses, schools, and the like. At the data rates of conventional modems, the transmission of detailed graphics, for example, typically requires a time that may be annoying to the user. A web page containing detailed graphics of 100 kilobytes may require 27 seconds for transmission. Optical fiber networks and CATV networks have sufficient bandwidth to permit high speed data transmission. However, the infrastructure is not presently available to provide data services to consumers on optical fiber or CATV networks on a widespread basis and at low cost.
An asymmetric digital subscriber line (ADSL) standard for data transmission is being developed to address these issues. Data transmission, according to the ADSL standard, permits transmission of simplex and duplex digital signals over the conventional twisted wire pairs that are used for plain old telephone service (POTS). The digital data signals are transmitted at frequencies above the baseband analog POTS band (0-4 kilohertz). The ADSL standard is a physical layer standard providing for a simplex downstream channel at a maximum rate of 6.2 megabits per second and a minimum rate of 1.544 megabits per second. The ADSL standard also includes a duplex digital channel at optional rates of 64 kilobits per second, 160 kilobits per second, 384 kilobits per second and 576 kilobits per second. The ADSL standard takes advantage of the fact that most consumer applications, such as Internet access, access to on-line information services, access to private networks and work-at-home applications, require a larger bandwidth into the home than out of the home. ADSL transport technology is described by R. Olshansky in "Moving Toward Low Cost Access to the Information Highway", Telephony, Nov. 7, 1994, pp. 31-37.
Another data service that is designed to take advantage of traffic asymmetry in upstream and downstream directions is the hybrid fiber coax (HFC) network. Telephone and cable companies are designing and constructing HFC networks, typically with a 750 megahertz downstream channel and a 25 to 35 megahertz upstream channel.
The transmission control protocol (TCP) is widely used for various data communication applications, including file transfer (FTP), remote login (telnet) and World-Wide Web (WWW). Data application performance is directly dependent on TCP throughput.
TCP provides reliable data communication by requiring acknowledgment of each data packet. The receiver sends back an acknowledgment packet containing an identifier (sequence number assigned by the sender) of the last byte that it successfully received. The lack of an acknowledgment indicates that either the packet was lost during the transmission or contained corrupted data upon arrival at the receiver. The acknowledgment can be incorporated into a data packet or can be placed in an acknowledgment packet of minimum size that carries no data. The first type of acknowledgment is referred to as a data-carrying acknowledgment packet, and the second as a minimum-size acknowledgment packet.
When TCP data packets arrive at the receiver faster than the acknowledgment packets are sent out, the receiver may use one packet to collectively acknowledge all data packets, instead of generating an acknowledgment for each data packet. This process is referred to as "cumulative acknowledgment."
In prior art networks, the acknowledgment, cumulative or noncumulative, is initiated by the receiver only. Intermediate nodes, such as routers and ADSL access devices, do not participate in the acknowledgment process. They merely forward the acknowledgment packets to the sender of the TCP data packets. However, the receiver is unaware of the asymmetric data channel beyond the local area network (LAN) and generates one acknowledgment packet for each data packet. The acknowledgment packet is sent on the LAN connection to a router. Therefore, cumulative acknowledgment does not occur when the receiver is not directly connected to the asymmetric data channel. The acknowledgment packets must be queued at the router for transmission on the slow upstream link. This causes the TCP throughput to be determined by the slow upstream link, since the sender is required to stop transmission and wait for the acknowledgment to arrive.
It is therefore desirable to eliminate the low throughput transmission of prior art networks and to allow TCP transmission to operate at the full speed of the data channel.